1. Field of the Invention
The present invention relates to a so-called, double-rod hydraulic shock absorber.
2. Description of the Related Art
Conventional hydraulic shock absorbers include a double-rod hydraulic shock absorber in which piston rods are installed on two sides of a piston. The piston rods of this double-rod hydraulic shock absorber are a main piston rod extending from the piston to one side, and a sub piston rod extending from the piston to the other side. The main piston rod extends through one of two oil chambers partitioned by the piston.
The sub piston rod extends through the other one of the two oil chambers. The main piston rod projects outside a cylinder from a first support member provided in one end portion of the cylinder. The sub piston rod extends through a partition in the cylinder, and is slidably supported by this partition. Therefore, the cylinder includes a first oil chamber positioned between the piston and first support member, and a second oil chamber positioned between the piston and partition. The first and second oil chambers communicate with each other via a passage in the piston. This passage includes a valve.
Conventional double-rod hydraulic shock absorbers of this type are described in, e.g., Japanese Patent No. 4996957 and Japanese Patent Publication No. 48-21039.
A hydraulic shock absorber described in Japanese Patent No. 4996957 includes piston rods in which the outer diameter of a sliding portion of a main piston rod is equal to that of a sliding portion of a sub piston rod. This hydraulic shock absorber also includes a pipe into which one end portion (this portion will be referred to as a sub piston rod hereinafter for convenience sake) of the piston rod extending through the above-described second oil chamber is inserted. The pipe includes a small-diameter portion and a large-diameter portion. The small-diameter portion has a diameter into which the sub piston rod is able to be inserted. One end portion of the small-diameter portion is fixed to a cover member to close the other end portion of the cylinder. The large-diameter portion is provided in the other end portion of the small-diameter portion. In this cover member, a communication path is provided that releases the internal air of the pipe to the atmosphere.
The large-diameter portion of the pipe is fitted in the inner circumferential surface of the cylinder. The large-diameter portion includes a ring-shaped guide member (second support member) defining the above-described partition, and a sealing member. The sealing member seals a portion between the second oil chamber and pipe.
A reservoir that compensates for temperature is provided between the large-diameter portion and cover member. When the volumes of the first and second oil chambers change due to a temperature change, or when the volume of the hydraulic oil changes, the reservoir replenishes the hydraulic oil to the second oil chamber or discharges the hydraulic oil therefrom.
The reservoir is defined by a third oil chamber communicating with the second oil chamber via a communication path, and a gas chamber partitioned from the third chamber by a free piston. The communication path that allows the second and third oil chambers to communicate with each other includes a recessed groove in the outer circumferential portion of the large-diameter portion of the pipe.
The free piston has a ring-shape, and is slidably fitted in the inner circumferential surface of the cylinder such that the pipe slidably extends through the free piston. A gas having a predetermined pressure is encapsulated in the gas chamber.
Like the hydraulic shock absorber disclosed in Japanese Patent No. 4996957, a hydraulic shock absorber described in Japanese Patent Publication No. 48-21039 includes a pipe into which a sub piston rod is inserted, and a reservoir that compensates for temperature. The pipe disclosed in Japanese Patent Publication No. 48-21039 is attached to a partition in a cylinder and to a cover member. The interior of the pipe is sealed. The partition is integral with the cylinder. The cylinder disclosed in Japanese Patent Publication No. 48-21039 is formed by threadably engaging two half portions split in the longitudinal direction. The partition is provided in the distal end portion of one half portion, and threadably engaged with the other half portion.
The partition includes a relief valve and a check valve. The relief valve opens to cause hydraulic oil to flow from a second oil chamber to a third oil chamber when the oil pressure of the second oil chamber rises. The check valve opens to cause the hydraulic oil to flow from the third oil chamber to the second oil chamber when the pressure of the second oil chamber becomes lower than a third pressure.
The hydraulic shock absorber disclosed in Japanese Patent No. 4996957 has the following difficulty in improving the ride quality of a vehicle when used in the vehicle.
The first problem is that a material having a high sealing ability must be used as the sealing member to seal the portion between the pipe and sub piston rod. This is because the interior of the pipe is released to the atmosphere. The sealing member must include the function of an oil seal to prevent leaking of the hydraulic oil from the second oil chamber, and the function of a dust seal to prevent the entrance of dust into the second oil chamber from inside the pipe. That is, the hydraulic shock absorber is disadvantageous in improving the ride quality of a vehicle because a sliding resistance generated when the sealing member comes into contact with the piston rod is high.
This inconvenience is able to be eliminated to some extent by improving the operability of the piston rod by making the dust seal unnecessary by sealing the interior of the pipe as disclosed in Japanese Patent Publication No. 48-21039. However, even the hydraulic shock absorber disclosed in Japanese Patent Publication No. 48-21039 cannot eliminate the following second problem.
In each of the hydraulic shock absorbers disclosed in Japanese Patent No. 4996957 and Japanese Patent Publication No. 48-21039, the outer diameter of the sliding portion of the main piston rod and that of the sliding portion of the sub piston rod are equal, so neither an expanding force nor a contracting force is generated. When using this hydraulic shock absorber in a suspension of a vehicle, a shock-absorbing rubber is interposed between one end portion of the hydraulic shock absorber and the body of the vehicle. A shock-absorbing rubber is also interposed between the other end portion of the hydraulic shock absorber and a member of the suspension, which moves together with a wheel with respect to the body of the vehicle. In each of the hydraulic shock absorbers disclosed in Japanese Patent No. 4996957 and Japanese Patent Publication No. 48-21039, neither an expanding force nor a contracting force is generated as described above, so no initial set load is provided to the shock-absorbing rubber. The initial set load is a load which elastically deforms the shock-absorbing rubber when the hydraulic shock absorber is not in operation. The second problem is that the responsiveness decreases because the hydraulic shock absorber expands or contracts after the shock-absorbing rubber deforms due to the displacement of the body of the vehicle. That is, since the hydraulic shock absorber operates later than the displacement of the body of the vehicle, the ride quality of the vehicle cannot be improved.
The initial set load is able to be provided to the shock-absorbing rubber by making the outer diameter of the sliding portion of the main piston rod smaller or larger than that of the sliding portion of the sub piston rod. By using this arrangement, the pressure-receiving areas of the first and second oil chambers of the piston become different from each other. The hydraulic oil is pressurized by the pressure of the gas in the gas chamber. Therefore, the piston is pushed toward the side having a smaller pressure-receiving area by a gas reaction force equivalent to the difference between the pressure-receiving areas. That is, the initial set load is provided to the shock-absorbing rubber because the hydraulic shock absorber expands or contracts in a natural state.
Unfortunately, this configuration cannot be applied to each of the hydraulic shock absorbers described in Japanese Patent No. 4996957 and Japanese Patent Publication No. 48-21039 for a structural reason. In this hydraulic shock absorber, if the outer diameter of the sliding portion of the main piston rod is made smaller or larger than that of the sliding portion of the sub piston rod, a large amount of hydraulic oil necessarily flows from the second oil chamber to the third oil chamber, or vice versa, as the piston moves. However, no sufficient passage area is able to be provided in each of the hydraulic shock absorbers described in Japanese Patent No. 4996957 and Japanese Patent Publication No. 48-21039.
That is, in the hydraulic shock absorber described in Japanese Patent No. 4996957, the communication path that allows the second and third oil chambers to communicate with each other includes a recessed groove in the outer circumferential portion of the large-diameter portion of the pipe. The ring-shaped guide member defining the partition is fitted in the inner circumferential portion of the large-diameter portion of the pipe. Therefore, the thickness of the pipe is small. It is difficult to provide a communication path having a sufficient passage area in this pipe. Since a large amount of hydraulic oil cannot flow through this communication path when the piston moves, cavitation may occur in the oil chamber. The communication path cannot have a large cross-sectional area by which no cavitation occurs, because the cross-sectional area is restricted by the thickness of the pipe.
In the hydraulic shock absorber described in Japanese Patent Publication No. 48-21039, the relief valve and the check valve are arranged between the second and third oil chambers. Accordingly, a large amount of hydraulic oil cannot freely flow between the second and third oil chambers of this hydraulic shock absorber.